Seismic Evaluation of Rcc Building with Softstorey at Ground Floor

Reinforced concrete frame buildings have become common form of construction with masonry infills in urban and semi urban areas in the world. The term infilled frame denotes a composite structure formed by the combination of a moment resisting plane frame and infill walls. Many such buildings constructed in recent times have a special feature - the ground storey is left open, which means the columns in the ground storey do not have any partition walls between them. There is significant advantage of this type of buildings functionally but from seismic performance point of view such buildings are considered to have increased vulnerability. Though multi-storeyed buildings with parking floor (soft storey) are vulnerable to collapse due to earthquake loads, their construction is still widespread. The open ground storey buildings are generally designed as framed structures without regard to structural action of masonry infill walls. The first storey becomes soft and weak relative to the other upper stories, since the first stories are composed of only the columns and upper stories are divided by the rigid walls. According to IS 1893 part I: 2002 a soft storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of average lateral stiffness of the three stories above. A extreme soft storey is one in which the lateral stiffness is less than 60 percent of that in the storey above or less than 70 percent of average lateral stiffness of the three stories above. It is found that, lateral displacement is very large in case of bare frame as compare to that of in-filled frames. If the effect of infill wall is considered then the deflection has reduced drastically. The presence of walls in upper storeys makes them much stiffer than open ground storey.